In many WLAN and other wireless data networks implementations, it is beneficial for the system owner to know the physical location of mobile clients or compatible tags. This will enable new features such as enhanced network security, providing of ‘location based’ services, asset tracking and many others.
A typical ‘location finding’ system, as currently implemented in the related patents and patent applications disclosed above, consists of multiple ‘location transceivers’ connected to the WLAN system, either by means of CAT-5 backbone or by wireless bridges. The typical ‘location transceiver’ contains a WLAN receiver and the circuitry required to extract Time of Arrival (TOA) information and report this information to the location server of the system. The ‘location server’ performs the required computation of the client or tag location based on the known location of the location transceivers, and displays it to the user or reports it to the requesting application.
In an IEEE 802.11a/b/g/n based Time Difference of Arrival (TDOA) location system, the TDOA of each pair of location transceivers is calculated from the reported TOA's that are calculated on a single 802.11a/b/g/n transmitted message. In a wireless local area data communication system, the Location Transceivers may be attached and/or integrated and/or be a part of the Access Points in said network.
The time synchronization of such a system using wireless or wired methods has also been previously done. Regarding wireless synchronization, U.S. Pat. No. 6,968,194 B2, entitled “METHOD AND SYSTEM FOR SYNCHRONIZING LOCATION FINDING MEASUREMENTS IN A WIRELESS LOCAL AREA NETWORK”, describes a location system in which multiple location receivers compute the time-of-arrival (TOA) of a reference transmitter signal, which is generally a beacon signal. The TOAs are collected and reported to a master unit that contains stored predetermined position information for the location receivers. The master unit computes the time-differences-of-arrival (TDOA) between multiple receivers and computes differences between the measured TDOAs and theoretical TDOAs computed in conformity with the predetermined position of each location receiver. The deviations between theoretical and measured TDOAs are collected in a statistical sample set and Kalman filters are used to produce a model of location receiver timebase offset and drift over multiple received beacon signals. The filter outputs are used to then either correct subsequent TDOA measurements for each location receiver, improving the accuracy of subsequent and/or prior TDOA measurements, or commands are sent to the location receivers to calibrate the timebases within the location receivers in order to improve the accuracy of subsequent TOA measurements.
The location accuracy of such a system is determined among many other factors by the accuracy of the TOA as calculated by the Location Transceivers and/or Access Points. The accuracy of the TOA, especially in multipath environments, is strongly affected by the bandwidth of the received signal and the limited bandwidth of the IEEE 802.11a/b/g/n signals is a strong limiting factor in achieving better location accuracy than 1-2 m.
In some location systems it is desirable to achieve an improved location accuracy compared to the accuracy achieved by IEEE 802.11a/b/g/n systems. It's well known that the location accuracy is strongly affected by the accuracy of the estimated TOA by each of the location transceivers, and the TOA maximum accuracy is mainly determined by the bandwidth of the received signal. Other factors as the signal SNR, time synchronization of the transceivers, modulation type, number of transceivers, etc. also affect the location accuracy.
Ultra wide band (UWB) systems have been designed to allow digital communication at very high data rates by using very wide spectrum bands (typically more than 500 MHz). Since the use of this huge band overlaps many other licensed and unlicensed bands, this technology has been limited to very low average transmission power (an average of less than −40 dBm/MHz) thus strongly limiting the communication range.
However, the use of those extremely wide bands is beneficial for time based location systems, since it allows a very accurate and precise TOA measurement in addition to an excellent separation of multipaths, even in the presence of very close multipaths (up to 20-30 nsec).
Therefore, a well designed UWB location system can achieve a typical location accuracy of ±1 feet while in several cases it's possible to achieve a location accuracy of few inches. However as previously mentioned, the range of such location systems is limited, in addition to other limitations as imposed by the regulatory bodies.
In such a location system (either IEEE 802.11a/b/g/n or UWB), a tag or standard client is required to transmit one or several messages to allow all those location transceivers or AP's to receive and measure the TOA of the transmitted messages. Those location transceivers maybe time synchronized by cables and/or over the air. The location transceivers calculate the TOA of each received message and report those values to a server which calculates the tag or mobile unit position. However, as stated above, both systems have some disadvantages.
Therefore, it would be desirable to provide a method and system to overcome the above problems. The system and method would provide a combined (IEEE 802.11a/b/g/n and UWB) location system in which the integration of both technologies provides a location system with significant advantages not present on each of those technologies separately.